The invention relates to a process for the biological purification of outgoing gas or waste water, in which the outgoing gas or waste water is conveyed through a gas-liquid contact apparatus. The invention also relates to an apparatus for carrying out the process.
The following requirements are imposed on biowashers and biofilters for the biological purification of outgoing air:
1. High efficiency, i.e. the most complete removal of pollutants and odorous substances from the outgoing air;
2. Small dimensions, in particular small cross-sectional area (space requirement);
3. Low energy consumption (operating costs);
4. Simple construction and operation;
5. Effectiveness for the widest range of pollutants in the greatest ranges of concentrations.
It is particularly difficult to satisfy these requirements if high requirements are imposed on the material exchange capacity owing to the high pollutant content of the outgoing air and the high oxygen requirement usually associated with it for biological decomposition or owing to the low water-solubility of the components to be removed.
During a detailed investigation of known biowashers and biofilters, it is found that all of them have considerable defects in at least one but usually in several of the above mentioned criteria.
Virtually all biofilters can be operated only at low gas speeds (-0.1m/s or lower) and therefore necessitate large areas. They tend to clog up if there is high pollutant content and an associated high growth of the biomass. If strong acids (HC1, HF, H.sub.2 SO.sub.4 or HNO.sub.3) are formed during biological decomposition as, for example, during the decomposition of halogenated hydrocarbons or various sulphur or nitrogen compounds, then the pH in the biofilters (for example compost filters) falls and the biological decomposition capacity comes to a standstill if the pH is not adjusted by addition of a base, and this increases the pressure loss and is accompanied by problems of poor distribution (liquid collects at edge with countercurrent) or losses in efficiency (liquid entrains pollutant).
The known biowashers, for example jet washers, packed columns or conventional plate columns (see, for example, Japanese patent application No. 51-67048) with subsequent bioreactors have the main disadvantage of the extremely high liquid stream from the washer to the bioreactor and back usually required for transporting dissolved harmful substances and dissolved oxygen. Although this disadvantage can be reduced by addition of sorbents (for example activated carbon) these measures are accompanied by new disadvantages (abrasion, maintenance and investment costs for coal storage and disposal).
The jet washer has the further disadvantage of the increasing complexity of the plant if low solubility of the pollutants and/or low permissible final concentration necessitate a multiple stage process.